Due to the goal of extensions being namespaced and living independently of each other, your settings must always contain your extension's namespace. BigTree will do this automatically for you when you create, update, delete, and retrieve the value of settings when within an extension.
If you wish to manually namespace, you can do so by referencing a setting ID with it's namespace. For instance, if you want to create a setting named "image-widths" you can reference it directly as "com.author.extension*image-widths" (where com.author.extension is your extension's ID). If you would rather have BigTree handle namespacing for you, simply reference the setting's id as "image-widths"and BigTree will prepend the extension namespace. If you choose to reference your setting in a file outside extension context (for instance, in a template not directly packaged into your extension) you MUST reference the fully namespaced ID (com.author.extension*image-widths).
The following calls have modified behavior when called from within an extension to support automatic prepending of the extension namespace: